1. Field of the Invention
The invention relates to an apparatus for inverting DC voltage to AC voltage by stacking of DC voltage sources from a series of DC voltage sources.
2. Background Art
There are many different types and sizes of inverters. The typical inverter arrangement has batteries that are hard connected in a series and/or parallel configuration, with a final set of battery connections delivering the resulting DC voltage to the inverter. This type of series/parallel battery arrangement makes for difficult battery maintenance because the heavy gauge, hard wired connections, and heavy current draw under load make it hard to diagnose an individual battery. If any one battery becomes defective, the series string that includes the defective battery will begin drawing current from a good string. This can be a difficult to detect situation, and results in reduced operational efficiency.
There are also several existing methods for stacking independent voltage sources in series for the purpose of power conversion. These power conversion approaches have been referred to using various names including multilevel converter, cascade inverter, cascade multilevel inverter, multilevel voltage source inverter, multilevel DC link inverter, and multilevel DC bus inverter. The differences among the various power conversion approaches have primarily to do with topologies, or the type of semiconductors used, and how they are connected.
These existing topologies for stacking independent voltage sources in series, although used for commercially successful applications, have certain disadvantages. Typically, all the power switching nodes (sub-circuits where H-bridges, or half-bridges, or discreet switching elements connect to the independent voltage sources used for any half AC wave output) must all be powered at all times while the inverter is generating AC voltage. In addition, the resulting current delivered to the load typically must pass through all of the switching nodes (or at least all the nodes used in any half AC wave) before being delivered to the load.
Further background information may be found in U.S. Pat. Nos. 5,642,275; 6,075,350; RE37,126; 6,577,087; and 6,969,967. Further background information may also be found in U.S. Pub. Nos. 2006/0044857; and 2009/0102436.
For the foregoing reasons, there is a need for an improved apparatus for inverting DC voltage to AC voltage.